1. Technical Field
The invention relates to methods for forming low pressure chemical vapor deposited silicon dioxide layers and more particularly to a chemical reactive deposition of silicon dioxide on a silicon nitride layer by reaction of a chlorosilane and an oxidizing gas at low pressure conditions.
2. Background Art
Silicon dioxide is widely used as an insulator and/or masking layer in the formation of semiconductor and integrated circuit devices. An important method for forming silicon dioxide is by the thermal growth method involving the thermal oxidation of silicon in the presence of oxygen or steam at relatively high temperatures above 900.degree. C. and usually 1000.degree. C. or more. The method is a high temperature technique which could cause a shift in the P-N junction boundary during the oxidation process. A further problem is that the method is only suitable when the silicon dioxide is to be grown on silicon. A further method for growing a silicon dioxide layer involves the use of a chemical vapor deposit reaction involving the gaseous oxidation of silicon tetrachloride, silane, dichlorosilane, or the like. This method has been used for many years at atmospheric pressure and more recently at low pressures of the order of less than 1 torr.
The atmospheric pressure method of chemical vapor deposition of silicon dioxide from a gaseous oxidation of silicon tetrachloride or silane has generally involved undesirably high temperatures such as between 900.degree. to 1200.degree. C. These temperatures would result in the problem of a shift of P-N junction boundaries which is undesirable. Low temperatures below 900.degree. C., gaseous oxidation of silane or silicon tetrachloride have resulted in poorer quality silicon dioxide films than the thermally grown silicon dioxide films. The M. J. Lim U.S. Pat. No. 4,002,512, entitled "Method of Forming Silicon Dioxide", dated Jan. 11, 1977, teaches a low temperature method for forming silicon dioxide films of high quality which utilizes dichlorosilane which is oxidized using an oxidizing gas such as O.sub.2, CO.sub.2, N.sub.2 O, H.sub.2 O, etc., combined with an inert carrier gas such as He, A, N.sub.2, H.sub.2, etc.
The use of a low pressure hot wall system for the chemical vapor deposition of silicon dioxide is described by J. Sandor presented at Electro-Chemical Society Meeting, Los Angeles, Calif., May 6-10, 1962, and by J. Oroshnik, et al, in Journal Electro-Chemical Society Solid State Science, Vol. 115, page 649, 1968, and by R. S. Rosler, entitled "Low Pressure CVD Production Processes for Poly, Nitride, and Oxide", in Solid State Technology, April 1977, pp. 63-70. The R. S. Rosler paper is a survey paper of the various low pressure chemical vapor deposited methods for forming polycrystalline silicon, silicon nitride and silicon dioxide films in the semiconductor industry. On page 68, Rosler describes the low pressure reactions of dichlorosilane and nitrous oxide in a temperature range of 800.degree. C. to 920.degree. C.
The gaseous reaction of chlorosilanes, and more particularly dichlorosilane, with nitrous oxide has been found to produce silicon dioxide layers which were of high quality in their refractive index, but were susceptible to degradation during subsequent oxidation cycles. The phenomena of degradation is believed to be an undercutting phenomena which results in flaking of the silicon dioxide layer that has been undercut. The resulting structures of this problem are as shown in FIGS. 1 and 2. FIG. 3 is a non-undercut plane view of a silicon dioxide layer formed by the present process.
Silicon nitride is an effective barrier to the penetration of impurities into the surface of a semiconductor body. U.S. Pat. No. 3,494,809 to C. A. Ross entitled "Semiconductor Processing" suggests the use of silicon nitride on the backside of semiconductor wafers during epitaxial growth on the front surface of the wafer, oxidation, etching and diffusion steps.